EP0747164B1 - Process for preparing brazing fluxes - Google Patents

Description

The present invention relates to a method for Preparation of a composition, the potassium salts of complex Contains fluorides of aluminum and as a flux suitable for soldering light metal materials such as aluminum is.

When brazing or furnace brazing, metal materials, especially aluminum, at a temperature above 500 ° C connected with the help of a molten filler metal (solder). The melting temperature of the solder is lower than that Melting temperature of the materials, so that these often through re-melting the solder can be separated non-destructively can.

When soldering, oxides and other disruptive cover layers a problem on the metal surface: the surface must be pure metallic, so that a perfect solder connection comes about. Flux is used for this, the usually brushed on, sprayed on or as a coating on the Material are applied.

Flux based on potassium salts of complex aluminum fluorides are particularly suitable as fluxes.

US-A 4,579,605 describes a process for the production a mixture of potassium tetrafluoroaluminate which can be used as a soldering flux and dipotassium pentafluoroaluminate or Hydrate, whereby aqueous fluoroaluminic acid (producible from Aluminum hydroxide and hydrofluoric acid) in one precipitation step with a converts aqueous solution of a potassium compound, the separating the resulting solid from the aqueous phase and the separated solid dries.

European patent application EP-A-0 063 750 (= US-A 4,428,920) describes a manufacturing process for a Flux. According to that process, fluoroaluminic acid mixed with a potassium compound in a precipitation step, preferably potassium hydroxide in the form of a potassium hydroxide solution, whereupon potassium salts of complex fluorides of aluminum fail. One then separates the water content in this Precipitating suspension of potassium salts of complex Fluorides of aluminum and dries the obtained Solids at 120 ° C. You get with this procedure a flux that melts below 575 ° C without leaving any residue.

That method advantageously uses freshly prepared ones Fluoroaluminic acid (can be made from alumina hydrate and hydrofluoric acid). The potassium compound is placed in the stoichiometric deficit; this means that in aqueous Supernatant of the complex aluminum fluoride precipitating fluoroaluminic acid is still present.

The object of the present invention is a method to indicate at which the starting compounds - at least consistent product properties especially in With regard to use as a soldering flux - better exploited become. This task is accomplished by the method of the present Invention solved.

The inventive method for producing an as Solder flux usable mixture of potassium salts complex Aluminum fluoride, which is a mixture of potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate or its hydrate contains, provides that aqueous fluoroaluminic acid in a precipitation step with an aqueous solution of a potassium compound implemented and in the precipitation stage a molar ratio of Potassium to aluminum in the range below 1: 1, which in of the precipitation stage resulting potassium cryolite suspension or Potassium cryolite-containing mixture is added, the resultant Solid separated from the aqueous phase and the separated solid dries, leaving the potassium cryolite or the mixture containing the potassium cryolite in one Adding the amount of suspension formed in the precipitation stage, that a molar ratio of potassium to the total batch Aluminum sets in the range of 1: 1 to 2: 1.

The fluoroaluminic acid advantageously becomes fresh prepared from alumina hydrate and hydrofluoric acid. The hydrofluoric acid can have a more or less large concentration of hydrogen fluoride contain, for example up to 60 wt .-%. It is expedient to use hydrofluoric acid with a concentration from 5 to 30 wt% hydrogen fluoride. The one available afterwards Fluoroaluminic acid can have an excess of fluoride. The ratio of fluorine to aluminum is preferably in the range from 4.0: 1 to 5: 1, very particularly preferably in Range from 4.0: 1 to 4.4: 1.

Potassium compounds are used which are used in the Reaction with fluoroaluminic acid to precipitate Potassium salts of complex fluorides of aluminum lead. Basic potassium compounds, their Anions dissolved or gaseous in the reaction from the in suspension resulting from the precipitation stage can be separated off, such as potassium hydroxide or potassium carbonate. Potassium hydroxide is preferably used, particularly preferably in the form of a Potassium hydroxide solution with a concentration of 2 to 25% by weight KOH. It is possible, although not preferred, part of the basic Potassium compound (e.g. potassium hydroxide) to be replaced by other potassium compounds such as potassium chloride; for example, up to a quarter of the potassium hydroxide in the form of potassium chloride or other potassium compounds deploy.

With respect to the preferred embodiment, namely the use of potassium hydroxide as the potassium compound the invention further explained, but without referring to the Use of potassium hydroxide should be restricted.

The implementation in the precipitation stage, i.e. during the reaction between fluoroaluminic acid and potassium hydroxide, can range from room temperature to the boiling point of the reaction mixture lie. Temperatures in the range of are preferred 70 ° C to 90 ° C. The temperature when adding the potassium cryolite is expedient to the boiling point, e.g. B. between 70 ° C and the boiling point, preferably in the range of 70 ° C to 90 ° C.

Pure K ₃ AlF ₆ can be used as the potassium cryolite. Technical products which contain only some of this compound (for example 50% by weight or more) can also be used. Other constituents of such technical compositions called "cryolite" are dipotassium pentafluoroaluminate and its hydrates, optionally also aluminum fluoride or its hydrates or potassium fluoride and other production-related impurities. A product called "cryolite" which can be used in the process according to the invention has, for example, the formula K _2.8 AlF _5.8 . The cryolite is preferably used in finely powdered form.

After the precipitation stage has been carried out, a post-reaction phase can take place be performed. For this you can use the reaction mixture advantageously stir. The duration is expediently 0.2 to 1 hour. The temperature is during the Post-reaction phase preferably in the range from 70 ° C to to the cooking point.

After the addition of the cryolite there can be a post-reaction phase be performed. The period is expedient in Range from 1 to 6 hours, the temperature is appropriate in the range from 70 ° C to the boiling point.

The following are explanations for the molar ratio added from potassium to aluminum to fluorine, with regard to on the corresponding ratio of the starting products in the precipitation stage, in relation to the corresponding ratio of the overall approach (i.e. taking into account the starting material in the precipitation stage plus the potassium cryolite used) and in relation to the corresponding molar ratio in the products received.

The amounts of starting materials (fluoroaluminic acid / Potassium compound) in the precipitation stage is preferably chosen such that a molar ratio of potassium: aluminum: fluorine in Range of 0.60-0.95: 1: 4-4.8 is observed. Is preferred a molar ratio of 0.80-0.90: 1: 4-4.4.

With regard to the molar ratio in the total batch, that is including the addition of potassium cryolite or equivalent Mixtures containing potassium cryolite, that is Molar ratio of potassium: aluminum: fluorine preferred 1.0 - 2: 1: 4-5, especially 1.1 - 1.2: 1: 4.4 - 4.8.

If the method according to the invention is carried out as above described by, after adding the cryolite, Separation of the existing water and drying or subjecting the product of a heat treatment a mixture of potassium salts of complex fluorides, which is very good as Soldering flux is useful and its molar ratio of potassium: aluminum: fluorine in the range of 1.1 - 1.3: 1: 4.1 - 4.3 lies. The product was found to be essentially from potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate or whose hydrate exists. Cryolite is at most in low Quantities demonstrated. The flux properties of the Materials can be improved even further if you - how in EP-A-0723835 described - the hydrate of dipotassium pentafluoroaluminate or only reversible when drying dehydrated dipotassium pentafluoroaluminate in irreversible dehydrated dipotassium pentafluoroaluminate. This is done by doing as described in EP-A-0723835 the mixture of potassium salts of complex aluminum fluorides if necessary predried and then a temperature treatment subject without the product sintering or melts. The term "temperature treatment" advantageously means that it is at a temperature above 228 ° C, preferably heated above 265 ° C until the desired Degree of irreversible dehydration is reached. On such a product leads to an even better flow behavior of the solder on the surface of metal materials.

If desired, this can be done with the invention Method obtained mixture, provided it as a soldering flux should be used, add metallic additives, as described in U.S. Patents 5,100,048 and 5,190,596 is. You mix a metal like silicon, copper or germanium preferably in the form of small particles, for example with a size of less than 1,000 microns, preferably in the range from 4 to 80 µm, too. The metal component is in the finished Flux then in an amount of about 9 to 85 wt .-% contain.

In the present proceedings used a reaction mixture after working up the desired mixture of potassium salts of complex aluminum fluorides, that as a flux for soldering light metal materials is suitable, supplies. This reaction mixture includes fluoroaluminic acid, potassium salts of complex Fluorides of aluminum and potassium cryolite. It is obtainable by reacting the reactant in the potassium cryolite precipitation stage adds.

The mixture obtainable in the process according to the invention of potassium salts of complex aluminum fluorides is particular as a flux for soldering light metal materials suitable as it has a melting point below 575 ° C. However, it can also be used for other purposes, for example as a filler in glass production or for abrasives in which experience has shown that such connection types can be used.

The present examples are intended to further advance the invention explain without restricting their scope.

Example 1:

614 kg of hydrogen fluoride in the form of a hydrofluoric acid containing 20% by weight of hydrogen fluoride and 550 kg of aluminum hydroxide (degree of purity 99%) were reacted to form fluoroaluminic acid. A potassium hydroxide solution containing 15% by weight of KOH was introduced into the reaction mixture obtained at a temperature of 80 ° C. and was obtained by dissolving 400 kg of KOH (degree of purity 90%). A suspension of precipitated potassium salts of complex aluminum fluorides was formed. 270 kg of a commercially available "cryolite" of the "formula" K _2.8 AlF _{5.8 were introduced} into the reaction mixture which had been obtained in the precipitation step described above.

The process product obtained contains KAlF ₄ and K ₂ AlF ₅ and has a melting point of 570 ° C. and is outstandingly useful as a soldering flux.

Example 2:

Production of a flux with heat treatment for the purpose Formation of irreversibly dehydrated dipotassium pentafluoroaluminate

Example 1 was repeated. The one obtained after adding cryolite The product was put in a filter drier wet. The inlet temperature in the dryer was around 570 ° C, the dwell time was about half a second.

The product obtained was obtained by X-ray diffractometry examined. It was found that in addition to predominantly Potassium tetrafluoroaluminate that irreversibly dehydrated Dipotassium pentafluoroaluminate was included. The melting point this product was around 570 ° C. When applying this Product in the form of an aqueous suspension resulted a very uniform coating of the workpiece, and that Solder had an even better flow behavior than with the flux of example 1.

Claims (6)

1. A method for the preparation of a mixture of potassium salts of complex aluminium fluorides which can be used as a soldering flux, which mixture contains a mixture of potassium tetrafluoroaluminate and dipotassium pentafluoroaluminate or the hydrate thereof, in which aqueous fluoroaluminic acid is reacted with an aqueous solution of a potassium compound in a precipitation stage, and in so doing a molar ratio of potassium to aluminium in the range of less than 1:1 is set in the precipitation stage, potassium cryolite or a mixture which contains potassium cryolite is added to the suspension produced in the precipitation stage, the resulting solid is separated off from the aqueous phase, and the solid separated off is dried, in which the potassium cryolite or the mixture containing potassium cryolite is added to the suspension produced in the precipitation stage in such a quantity that a molar ratio of potassium to aluminium in the range of 1:1 to 2:1 is set for the total formulation.
2. A method according to Claim 1, characterised in that fluoroaluminic acid which has a molar ratio of fluorine to aluminium in the range of 4:1 to 5:1 is used.
3. A method according to one of the preceding claims, characterised in that the reaction in the precipitation stage is carried out at a temperature up to the boiling point of the reaction mixture, preferably at a temperature of 70 to 90°C, and upon the addition of the potassium cryolite or the mixture containing potassium cryolite at a temperature from 70°C up to the boiling point.
4. A method according to one of the preceding claims, characterised in that the dewatered solid is subjected to high-temperature treatment such that at least part of the afore-mentioned complex aluminium fluorides is converted into irreversibly dehydrated dipotassium pentafluoroaluminate.
5. A method according to Claim 1, characterised in that potassium hydroxide in the form of potassium hydroxide solution is used as the potassium compound.
6. A method according to Claim 1, characterised in that the cryolite is used in finely powdered form.